The subject matter described herein relates to electronic communication, and more particularly to signal encoding techniques which may be used in wireless communication systems such as satellite communication systems. In addition, the subject matter described herein relates to techniques to perform geolocation using low-earth orbit (LEO) satellite signals.
The Global Positioning System (GPS) is a space-based, world-wide navigation system which includes a space, ground, and user segment. The locations of the satellites are used as reference points to calculate positions of the GPS user receiver, which is usually accurate to within meters, and sometimes even within centimeters. Each of the satellites, the ground stations, and the GPS user receiver has preprogrammed timing signals that start at precise times. In order to lock on to the signals broadcasted by the satellites, the ground station and GPS user receiver slew their respective internal generated signals relative to time as predicted by their respective internal clocks. When the signals are locked, the GPS user receiver makes ranging measurements to each satellite called pseudoranges. These pseudorange measurements include the actual ranges to the satellites, in addition to an error associated with the receiver clock time offset relative to GPS time, plus other smaller errors. The ground stations included in the GPS control segment network provide ranging measurements which are used to generate predictions for the satellites clocks and orbits. These predictions are periodically uploaded to the satellites and the satellites broadcast this data to the user receiver to support the user receiver positioning function.
Due in part to concerns about jamming of GPS signals and power limitations on GPS signals, low Earth orbiting (LEO) satellite constellations, such as Iridium, have been suggested as a mechanism to allowing navigation without using GPS, or in conjunction with GPS systems. The Iridium satellite constellation is a constellation of low earth orbiting satellites at a height of approximately 485 mi (781 km), with six near polar orbits with an inclination of 86.4°, and maintains an orbital velocity of approximately 17,000 mph (27,000 km/h). The constellation is used to provide global satellite communication services including voice and data coverage to satellite phones, pagers, and integrated transceivers around the globe. The constellation includes sixty-six active satellites in orbit which orbit from pole to pole at an orbital period of just over 100 minutes. The orbit design produces counter-rotating planes of satellites next to one another traveling in opposite directions. In addition, the satellites use cross-linking technology to relay data across the constellation. Each Iridium satellite has 48 spot beams as part of the constellations uniquely-identifiable beam geometry that project onto the surface of the earth.
Various techniques for using the time standard from the LEO satellites to augment the functionality of the GPS system include such patents as U.S. Pat. RE 37,256, issued to Cohen, et al. entitled, “System and Method For Generating Precise Position Determinations;” U.S. Pat. No. 5,812,961 issued to Enge, et al. entitled, “Method And Receiver Using A Low Earth Orbiting Satellite Signal To Augment The Global Positioning System;” U.S. Pat. No. 5,944,770 issued to Enge, et al. entitled, “Method And Receiver Using A Low Earth Orbiting Satellite Signal To Augment The Global Positioning System;” and U.S. Pat. No. 6,373,432 issued to Rabinowitz, et al. entitiled “System Using LEO Satellites For Centimeter-Level Navigation.” The disclosures of these patents are incorporated herein by reference in their respective entireties.
The Iridium system employs a signal processing scheme known as differential encoding to encode and transmit quadrature phase shift keyed (QPSK) modulated broadcasts. During the differential encoding process the In-Phase and Quadrature modulated bits (Is and Qs) of a broadcast are re-coded as such that the output state of the differential encoder is a function of both the current (I,Q) input to the differential encoder and the previous (I,Q) state of the differential encoder. This differential encoding scheme raises issues when using pseudo-random code sequences and associated correlation detection methods because for every desired code that is intended to transmit there are multiple possible broadcasts. This complicates the correlation processing in the user receiver. Instead of searching and correlating for a group of possible code messages the receiver instead must search through a multiple of the number of messages broadcast. This increases the memory, processing, and power requirements of the user receiver. Furthermore, it weakens the separation between the codes and increases the likelihood that one code might be mistaken for another in the correlation process.
Accordingly, additional encoding techniques for use with LEO satellite systems such as Iridium may find utility.